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United States Patent |
6,123,803
|
Genut
,   et al.
|
September 26, 2000
|
Laser processing chamber with cassette cell
Abstract
A process chamber for carrying out laser treatments, on the surface of an
object, comprising: a base provided with object support means; a cover
provided with a window substantially transmissive of laser light; gas
inlet and gas outlet means; the said cover and the said base, when
connected, leaving a space between the surface of the element and the
inner surface of the window, in which gases flowing through the said gas
inlet may flow above the surface of the object being treated and out of
the process chamber through the said gas outlet.
Inventors:
|
Genut; Menachem (Haifa, IL);
Livshit (Buyaner); Boris (Carmiel, IL);
Tehar-Zahav; Ofer (Natania, IL)
|
Assignee:
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Oramir Semiconductor Equipment Ltd. (Haifa, IL)
|
Appl. No.:
|
068058 |
Filed:
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April 28, 1998 |
PCT Filed:
|
November 4, 1996
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PCT NO:
|
PCT/IL96/00141
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371 Date:
|
April 28, 1998
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102(e) Date:
|
April 28, 1998
|
PCT PUB.NO.:
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WO97/17167 |
PCT PUB. Date:
|
May 15, 1997 |
Foreign Application Priority Data
Current U.S. Class: |
156/345.5; 134/1.3; 219/121.86; 438/795 |
Intern'l Class: |
C23F 001/02 |
Field of Search: |
156/345
134/1.3
438/690,795
219/121.6,121.68,121.86
118/723
|
References Cited
U.S. Patent Documents
4752668 | Jun., 1988 | Rosenfield et al.
| |
5024968 | Jun., 1991 | Engelsberg.
| |
5099557 | Mar., 1992 | Engelsberg.
| |
5114834 | May., 1992 | Nachshon.
| |
Foreign Patent Documents |
350021 | May., 1993 | EP.
| |
661110 | Jul., 1995 | EP.
| |
62-276828 | Oct., 1987 | JP.
| |
1018226 | Jan., 1989 | JP.
| |
2165616 | Jun., 1990 | JP.
| |
WO 93/19888 | Oct., 1993 | WO.
| |
WO 94/23854 | Oct., 1994 | WO.
| |
WO 95/07152 | Mar., 1995 | WO.
| |
WO 96/09128 | Mar., 1996 | WO.
| |
Primary Examiner: Bueker; Richard
Assistant Examiner: Powell; Alva C.
Attorney, Agent or Firm: Cowan, Liebowitz & Latman, P.C., Dippert; William H.
Claims
What is claimed is:
1. A process chamber for carrying out laser ablation/etching as well as
combustion and evacuation of foreign materials from substrate surfaces, in
ambient gas flow of two reactive gas components, which comprises:
(a) a base provided with object support means;
(b) a cover provided with a window substantially transmissive of laser
light;
(c) an irradiation zone;
(d) a stagnation chamber;
(e) separate inlets into said stagnation chamber for said two components of
the reactive gas;
(f) a reactive gas outlet;
(g) an outside vacuum channel;
(h) a first seal defining a first vacuum zone in the process chamber; and
(i) a second seal defining with said first seal a second vacuum zone
defining the pressure in said outside vacuum channel.
2. A chamber according to claim 1, associated with means for maintaining in
said second zone a pressure lower than in said first zone.
3. A chamber according to claim 1, associated with the said outside vacuum
channel, for avoiding the damage of leaking the hazardous reactive process
gases to the said outside working environment.
4. A chamber according to claim 1, having structural features and
dimensions adapted for laser ablation, etching, cleaning and other laser
surface treatments in ambient reactive gas including fast combustion and
fast gas flow.
5. A chamber according to claim 1, wherein the product of the pressure (P)
and of the distance (h) between the surface of the object to be treated
and the inner surface of the window is in the range of 40-60
Pa.multidot.m.
6. A chamber according to any one of claims 1 to 3, wherein the distance
between the surface of the object to be treated and the inner surface of
the window is in the range 0.2-10 mm.
7. A chamber according to claim 1, wherein the distance between the surface
of the object to be treated and the inner surface of the window is about 2
mm.
8. A chamber according to claim 1, wherein the window is made of a material
chosen from among silica quartz, MgF, CaF and sapphire.
9. A chamber according to claim 1 or 2, wherein the base and the cover of
the chamber are made of a material selected from among quartz, stainless
steel, aluminum or ceramic materials.
10. A chamber according to claim 1, wherein pressurization is obtained by
means of sealing rings.
Description
FIELD OF THE INVENTION
The present invention relates to U.V. laser surface treatment methods,
particularly to the removal of foreign materials from substrate surfaces.
More particularly, the invention relates to a process chamber for the
aforesaid purposes, which provides effective dry laser stripping and
cleaning.
BACKGROUND OF THE INVENTION
In the manufacturing of various products it is necessary to apply a layer
of protective material on a surface, which must be removed after a
specified manufacturing step has been concluded. An example of such
process is the so-called "masking", where a pattern is created on a
surface using a layer of protective material illuminated through a mask,
and the surface is then treated with a developer which removes material
from the unmasked portions of the surface, therefore leaving a
predetermined pattern. The surface is then treated by ion implantation or
by etching agents, which introduce the implanted species into the unmasked
portions of the surface, or removes material from unmasked portions. Once
these processes are completed, the role of the protecting mask ends and it
must be removed. The process is conventional and well known in the art,
and is described, e.g., in U.S. Pat. No. 5,114,834.
Two main photoresist stripping methods exist in the modern VLSI/ULSI
(Very/Ultra Large Scale Integration) circuits industry:
1) Wet stripping which uses acids or organic solvents;
2) Dry stripping, which uses plasma, O.sub.3, O.sub.3 /N.sub.2 O or
U.V./O.sub.3 -based stripping.
Both methods are problematic and far from being complete, especially when
taking into consideration the future miniaturization in the VLSI/ULSI
industry. The current technology is capable of dealing with devices having
feature sizes of about 0.5 .mu.m, but before the end of the century the
expectation is that the workable size of the devices is expected to be
reduced to 0.25 .mu.m. The expected size changes require considerable
changes in the manufacturing technology, particularly in the stripping
stage. The prior art photoresist stripping techniques described above will
be unsuitable for future devices, as explained hereinafter.
Utilizing only the wet stripping method is not a perfect solution, as it
cannot completely strip photoresist after tough processes that change the
chemical and physical properties of the photoresist in a way that it makes
its removal very difficult. Such processes include, e.g.,. High Dose
Implantation (HDI), reactive Ion Etching (RIE), deep U.V. curing and high
temperatures post-bake. After HDI or RIE the side walls of the implanted
patterns or of the etched walls are the most difficult to remove.
In addition, the wet method has some other problems: the strength of
stripping solution changes with time, the accumulated contamination in
solution can be a source of particles which adversely affect the
performance of the wafer, the corrosive and toxic content of stripping
chemicals imposes high handling and disposal costs, and liquid phase
surface tension and mass transport tend to make photoresist removal uneven
and difficult.
The dry method also suffers from some major drawbacks, especially from
metallic and particulate contamination, damage due to plasma: charges,
currents, electric fields and plasma-induced U.V. radiation, as well as
temperature-induced damage, and, especially, incomplete removal. During
various fabrication stages, as discussed above, the photoresist suffers
from chemical and physical changes which harden it, and this makes the
stripping processes of the prior art extremely difficult to carry out.
Usually a plurality of sequential steps, involving wet and dry processes
are needed to remove completely the photoresist.
The art has addressed this problem in many ways, and commercial photoresist
dry removal apparatus is available, which uses different technologies. For
instance, UV ashers are sold, e.g. by Hitachi, Japan (UA-3150A), dry
chemical ashers are also available, e.g., by Fusion Semiconductor Systems,
U.S.A., which utilize nitrous oxide and ozone to remove the photoresist by
chemical ashing, microwave plasma ashing is also effected, e.g., as in the
UNA-200 Asher (ULVAC Japan Ltd.). Also plasma photoresist removal is
employed and is commercially available, e.g., as in the Aspen apparatus
(Mattson Technology, U.S.A.), and in the AURA 200 (GASONICS IPC, U.S.A.).
More recently, photoresist removal has been achieved by ablation, using
laser UV radiation, in an oxidizing environment, as described in U.S. Pat.
No. 5,114,834. The ablation process is caused by strong absorption of the
laser pulse energy by the photoresist. The process is a localized ejection
of the photoresist layer to the ambient gas, associated with a blast wave
due to chemical bonds breaking in the photoresist and instant heating. The
partly gasified and partly fragmented photoresist is blown upwards away
from the surface, and instantly heats the ambient gas. Fast combustion of
the ablation products occurs, due to the blast wave and may also be due to
the photochemical reaction of the UV laser radiation and the process
gases. The main essence of the process is laser ablation with combustion
of the ablated photoresist which occurs in a reactive gas flowing through
an irradiation zone. The combination of laser radiation and fast
combustion provides instantaneous lowering of the ablation threshold of
hard parts of the photoresist (side walls). The combusted ablation
products are then removed by vacuum suction, or by gas sweeping leaving a
completely clean surface.
The aforementioned patent U.S. Pat. No. 5,114,834 does not describe any
particular requirements for the ablation cell, which is referred to as a
"container" or a "process chamber". However, it has been found that the
structure of the process chamber has a critical effect on the quality of
the stripping process.
While reference will be made throughout this specification to the ablation
of photoresist from semiconductor wafers, this will be done for the sake
of simplicity, and because it represents a well known and widely
approached problem. It should be understood, however, that the invention
described hereinafter is by no means limited to the stripping of
photoresist from wafers, but it applies, mutatis mutandis, to many other
applications, such as stripping and cleaning of photoresist from Flat
Panel Displays (FPD) or removal of residues from different objects, such
as lenses, semiconductor wafers, or photo-masks.
SUMMARY OF THE INVENTION
It has now been found, and this is an object of the invention, that not
every dimension and configuration of the process chamber for carrying out
pulsed U.V.-laser ablation/etching of foreign materials from substrate
surface in ambient reactive gases, provides goods results and that in
order to obtain satisfactory results there are certain dimensional
constraints to be observed. These constraints are important as they
provide conditions of reactive gas decomposition and excitation for
stripping and fast enhanced combustion of the ablation products.
The invention provides a stripping apparatus employing a process chamber
where the laser ablation/etching takes place in a cassette cell
configuration in ambient reactive gases, which can be used for the
effective stripping or removal of coatings, such as photoresist, to yield
a cleaned product of high quality.
DETAILED DESCRIPTION OF THE INVENTION
The process chamber for carrying out pulsed U.V.-laser ablation processes
on the surface of an object in ambient reactive gases, according to the
invention, comprises:
a base provided with object support means;
a cover provided with a window substantially transmissive of laser light;
reactive gas inlet and reactive gas outlet means; the said cover and the
said base, when connected, leaving a space between the surface of the
element and the inner surface of the window, in which gases flowing
through the said gas inlet may flow above the surface of the object being
treated and out of the cell through the said gas outlet.
In order to obtain optimal results, fast ignition and combustion of
ablation products are needed. This is needed in order to achieve maximal
and complete burning, volatilization and removal of such products during
the short interval between laser pulses (about 10.sup.-2 s). According to
a preferred embodiment of the invention, this is achieved by providing a
cell in which the product of the total pressure in the cell, (P) and of
the distance (h) between the surface of the object to be treated and the
inner surface of the window (hereinafter referred to as "gap" (h)), is
approximately constant. This means maintaining the same amount of oxidizer
needed for combustion as required by the stoichiometry. Mathematically, it
can be expressed as P.multidot.h=K. K is constant for a given set up of
ablation parameters as laser fluence energy at a given wavelength. A
typical range of K is 40-60 Pa.multidot.m or N.multidot.m.sup.-1 with an
average value of 50 Pa.multidot.m.
For a typical working pressure of 250 mbar or 2.multidot.5.times.10.sup.4
Pa, the gap h can be calculated as h=k/P=50/2.5.times.10.sup.4
=2.times.10.sup.-3 m=2 mm.
A variety of construction materials can be employed in the construction of
the process chamber of the invention. According to one preferred
embodiment of the invention, the window is made of fused silica, quartz,
MgF.sub.2, CaF.sub.2 and sapphire, or the like material. According to
another preferred embodiment of the invention, the base and the cover of
the cell are made of a material selected from among quartz, stainless
steel (e.g., 316 L), and aluminum, preferably "hard-anodized", or from
ceramic materials, e.g. alumina.
Pressurization of the cell can be obtained by any suitable means known in
the art, e.g., by means of sealing rings, such as O-rings.
The above and other characteristics and advantages of the invention will be
better understood through the following illustrative and non-limitative
description of preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a general side view of a process chamber according to one
preferred embodiment of the invention, some internal functional elements
being outlined in broken lines;
FIG. 2A is a cross-sectional view of the chamber of FIG. 1, taken along the
2--2 plane and FIGS. 2B and 2C are details of FIG. 2A;
FIG. 3 is an enlarged cross-section of the gas inlet assembly, taken along
the 3--3 plane of FIG. 2;
FIG. 4 is a bottom view of the process chamber of FIG. 1; and
FIG. 5 is a top view of the process chamber of FIG. 1.
DETAILED DESCRIPTION OF THE DRAWINGS
Looking now at FIG. 1, numeral 1 generally indicates a process chamber
consisting of a base 2, and a cover 3, which are connected by air-tight
connections (not shown), so that the inner part of the process chamber
defined by the said base 2 and cover 3 can be kept under pressure or
vacuum. The base 2 is provided with N.sub.x O.sub.y gas inlet 4, and
O.sub.2 /O.sub.3 inlet (shown in FIG. 3), which will be further described
below, and gas outlet 5, for exhausting gases which have passed through
the irradiated zone. The base 2 is further provided with a chuck 6, on
which the element to be stripped, e.g., a wafer, is supported. Through the
center of chuck 6 vacuum is applied, to hold the wafer in place during the
process.
Looking now at the parts shown in broken lines, the inlet gas and
stagnation cell 7 is seen, for the introduction and mixing of inflowing
gases, as well as the exhaust assembly 8. A fused silica window 9 is
provided above the element to be stripped, e.g., a silicon wafer, to which
reference will be made in the following description for the sake of
simplicity. This window permits the passage of the laser beam which
originates from a source positioned above the chamber 1. A cover frame 10
keeps the silica windows in place, and assists in keeping the chamber
pressurized or under vacuum.
FIGS. 2A-2C show in greater details and in cross-section, some of the
elements of the process chamber of FIG. 1. Two seals, 11A and 11B are
shown in this cross-section, which may be, e.g., O-rings. These two seals
define two vacuum zones in the process chamber:
a) Zone 1, which defines the ablation environment in the irradiation zone.
The pressure is maintained by means of throttle valve connected in a
closed loop to a pressure controller. Typical pressure is in the range of
50-2000 mbar. this pressure regime is defined by seal 11B.
b) Zone 2, which defines the pressure in the outside vacuum channel 20 in
between seals 11A and 11B.
The pressure in the channel is always much lower than in the process
chamber and usually is in the order of a few millibars or typical vacuum
obtainable from mechanical vacuum pumps.
The outer vacuum channel 20 has two main purposes:
1. To maintain firmly cover 3 through the aid of the atmospheric pressure.
2. For safety purposes, to avoid the possibility of leaking of hazardous
process gases through seal 11B. Here, in case of a leak, the gas will be
sucked by the vacuum pump connected to channel 20.
The wafer is positioned above chuck 6 and below window 9, as indicated by
numeral 12. Wafer 12 can be positioned on chuck 6 in two ways, as shown in
FIGS. 2B and 2C. In FIG. 2B the wafer is on top of chuck 6. In FIG. 2C the
wafer is immersed inside chuck 6. As stated, the wafer does not touch
window 9, and there is a distance between them which is preferably kept in
the range of 0.2-10 mm. As explained, this distance can be varied as long
as the product of the values of P.times.h remains approximately constant,
wherein P is the pressure above the wafer and h is the gap, as
hereinbefore defined. The pressure referred to above is measured in the
center of the process chamber in the irradiation zone.
The space between the window and the wafer defines the ablation cassette
cell, through which the gases flow, and in which the ablation products are
jetted from the wafer, ignited and combusted. Looking at gas inlet
stagnation cell 7, it can be seen that the inflowing gases flow into the
ablation cell through a communication opening indicated by numeral 13.
The window above the wafer is made of fused silica, because it must fulfill
certain requirements such as optical quality, to permit maximum passage of
the incident laser beam (indicated in the figure by the LB arrow),
durability, resistance to process gases and temperature, mechanical
strength, etc. However, it is clear that alternative materials can be
used, as long as they meet the desired operational conditions.
FIG. 3 shows the gas inlet stagnation cell, according to one embodiment of
the invention. The stagnation cell is shown in enlarged partial
cross-section, taken along the C--C plane of FIG. 2. The stagnation cell
comprises gas inlet 4. consisting of three separate inlets, two inlets 14
and 14', for N.sub.x O.sub.y gas, and one inlet 15, for O.sub.2 /O.sub.3
gas. The gas generator, as well as the O.sub.3 or N.sub.x O.sub.y
generator, if any, are located near the process chamber. As it can be seen
from the figures, the gases enter separately into the process chamber and
are mixed only while the O.sub.2+ O.sub.3 gas passes the outlet of the
N.sub.x O.sub.y nozzle 17. The N.sub.x O.sub.y is introduced through small
holes 18.
FIG. 4 shows the bottom of the process chamber of FIG. 1, from which the
bottom of the chuck 6 can be seen, as well as the exhaust 5 and inlets 14,
14' and 15 of the gas inlet assembly. It should be noted that two inlets
are provided in this embodiment for N.sub.x O.sub.y, while only one inlet
is provided for O.sub.3. However, different gas inlets can of course be
used, as long as the gases are introduced separately, without exceeding
the scope of the invention.
FIG. 5 shows a top view of the process chamber of FIG. 1, from which the
wafer 12 can be seen below the window 9. According to the particular
embodiment of FIG. 5, the wafer and the cover frame are positioned
asymmetrically with respect to the cover. This is done in order to permit
a laser cleaning while scanning of the outlet channel 8 where some
ablation residues may accumulate.
It should be understood that the process chamber of the invention is not
limited to be used in any particular apparatus, and it can be used in
ablation stripping and cleaning to be hereinafter mentioned as laser
treatment processes in any suitable system. For instance, the process
chamber can be kept stationary while the laser beam scans the wafer, or
the laser beam can be fixed and the process chamber can move by means of a
suitable X-Y system. Additionally, the invention is not limited to any
particular shape or size of process chamber, and can be used to perform
laser treatment processes on much larger or much smaller surfaces, can be
of different shapes, and can employ different construction materials.
Furthermore, the cell can be utilized for processes employing a variety of
gases, and is by no means limited to the use with the gases exemplified in
the above illustrative and non-limitative description of preferred
embodiments.
One version of the process chamber can be utilized for special purposes,
where the wafer or object to be cleaned or stripped is maintained at an
elevated temperature to assist and enhance the chemical process. Typical
temperatures can be between 75-350.degree. C. The heat source can be
outside the chamber, and heat may be provided, e.g., by radiation, or it
may be inside and heat may be provided by conduction. In some applications
it is also possible to introduce the gases at elevated temperature, to
maintain the desired temperature in time. It is also possible to heat the
outlet channel 8 to temperatures between 150-350.degree. C., to assist the
combustion of the accumulating ablation products by reactive process
gases.
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